For many years, it has been recognized that metals, plastics and other medium and low strength materials could be reinforced with silicon carbide whiskers. These whiskers are small fibers, which are mixed with the powdered metal or plastic to form a composite material.
The silicon carbide whiskers are obtained by well-known processes, such as by reacting coked rice hulls, a throw-away byproduct of rice production, as is taught in U.S. Pat. No. 3,754,076, issued Aug. 21, 1973 to Cutler.
The whiskers being fibers with anisotropic properties, have properties which are used to greatly reinforce and improve the mechanical properties of other materials.
While the advantages of utilizing these whiskers are well-known, it has been very difficult to properly mix these whiskers with other substances due to their initial agglomerated state. The mass of agglomerated whiskers when mixed with other materials will produce a non-homogeneous composite of lower mechanical strength.
In order to achieve the maximum benefits of whisker reinforcement, it is necessary to thoroughly and uniformly disperse and/or distribute the whiskers throughout the matrix material. Such dispersion requires thorough deagglomeration and mixing without substantial damage to the whiskers.
It has been taught in the prior art that mixing whiskers with matrix material and a non-polar solvent containing a polymeric binder can provide a random distribution of whiskers in the matrix. Such a teaching is shown in U.S. Pat. No. 4,259,112, issued Mar. 30, 1981 to Doloruy et al.
While U.S. Pat. No. 4,259,112 asserts being able to achieve a homogeneous composite, this homogeneity is not sufficient to provide a material (for a given whisker content) having the superior mechanical properties of this invention. For example, for a 2024 aluminum composite containing 20 percent by volume SiC whiskers, the above patent lists an ultimate tensile strength of 55.4 kpsi, whereas the inventive composite has an ultimate tensile strength (in a naturally aged temper) of approximately 100 kpsi.
Expressed in another way, since U.S. Pat. No. 4,259,112 does not teach deagglomerating the whisker mass prior to mixing, the product disclosed will be a composite containing agglomerates. These agglomerates seriously detract from the mechanical performance.
It has been discovered by the present inventors that the agglomerated whisker mass can be deagglomerated prior to mixing by treating it with a highly polar solvent and milling the solvated mass into a deagglomerated slurry. Alcohols, water and halogenated hydrocarbons are good polar solvents for this purpose. These directionally charged solvents are believed to work by reason of their ability to neutralize the statically charged whiskers of the agglomerated mass. This neutralization allows the whiskers to separate, i.e. deagglomerate, upon subsequent milling of the slurry.
When a polar solvent slurry of deagglomerated whiskers is then mixed with matrix material, such as powdered aluminum, a highly homogeneous, uniformly dispersed whisker composite is achieved, which is free of agglomerates. The deagglomerated whiskers are easier to mix with matrix materials due to the prior deagglomeration.
While treatment with non-polar solvents may be useful in assisting mixing by way of solvating the whisker mass, it does not provide the higher degree of uniformity and homogeneity achieved with the use of polar solvents and does not eliminate agglomerates. This is so because, prior deagglomeration of the whisker mass is not achieved without charge neutralization and milling.
The above deagglomerating process has produced aluminum composites of very high ultimate strengths. This is the first time to the best of our knowledge and belief, anyone has been able to consistently achieve an aluminum and silicon carbide whisker composite approaching an ultimate strength of 100 kpsi.